Regarding a translucent alumina and its production method, a high temperature sintering method in hydrogen or vacuum atmosphere, a pressure sintering method using HIP or the like, and the like are conventionally known.
Regarding the former, there are many reports such as a method of adding a grain growth inhibitor such as MgO to an alumina powder and sintering the mixture at high temperature of 1,600° C. or more in a hydrogen-containing atmosphere or a vacuum atmosphere (for example, see Patent Document 1). In this method, a translucent alumina having flexural strength of from 300 to 400 MPa comprising large crystal grains having a grain size of from about 20 to 50 μm is produced.
On the other hand, the latter method is a method of pressureless sintering a high purity alumina powder and highly densifying the same with HIP treatment to impart translucency (see Patent Documents 2 to 8). In this method, a sintering temperature of 1,500° C. or lower is used. Therefore, a sintered body comprising fine grains of from about 0.5 to 5 μm can be produced, and a translucent alumina having high flexural strength of 500 MPa or more is obtained. Furthermore, there is reported that by decreasing the grain size to 2 μm or less, flexural strength (about 800 MPa) which is about two times that of the translucent alumina produced by the conventional high temperature sintering method is obtained (see Patent Documents 2, 4, 5 and 8). However, those translucent aluminas have high flexural strength, but had the problem that fracture toughness is low, and therefore the alumina is brittle and is liable to get chipped. For example, Patent Document 3 reports a low fracture toughness value of from 3 to 4 MPa·m0.5.
A high toughness sintered body having fracture toughness of from about 5 to 9 MPa·m0.5, which is a sintered body having an anisotropy in grain shape and comprises slender grains is reported as an alumina sintered body having high fracture toughness (Non-Patent Document 1). However, this sintered body has high toughness but did not have translucency. The reason for this is considered that the presence of impurities and/or additives of from 500 to several thousand ppm is necessary in the conventional sintered body of anisotropic grains, and this impairs sintering properties. For example, the case that when additives are added to a high purity translucent alumina by HIP pressure sintering method to form slender anisotropic crystal grains, the sintered bodies are opaque is reported in Patent Document 5, and it is indicated that anisotropic grains do not result in translucency.
There are only the reports that a translucent alumina by the conventional HIP pressure sintering method comprises isotropic crystal grains, and this is apparent from texture photographs of sintered body in the reports up to now (see Patent Documents 3 to 8).
Thus, in the conventional non-pressure high temperature sintering method, only a translucent alumina of low strength and low toughness is obtained, and in the HIP pressure sintering method, only a translucent alumina of low toughness although high strength is obtained. Thus, an alumina sintered body having high strength/high toughness and high translucency in combination was not obtained.
Patent Document 1: U.S. Pat. No. 3,026,210
Patent Document 2: JP-A-63-236757
Patent Document 3: JP-A-3-168140
Patent Document 4: JP-A-3-261648
Patent Document 5: JP-A-2001-322866
Patent Document 6: U.S. Pat. No. 6,878,456
Patent Document 7: U.S. Pat. No. 6,648,638
Patent Document 8: JP-A-2006-87915
Non-Patent Document 1: J. Ceram. Soc. Jpn., 106 [12], pp. 1172-77 (1998)